1. Field of the Invention
The present invention relates to methodology for controlling the residual stresses in thin film materials formed by deposition on a substrate, and also relates to thin film structures which are characterized by reduced stress therein relative to thin films of corresponding materials produced by the prior art.
2. Description of the Related Art
In the prior art fabrication of thin film materials such as metal-containing films formed by vapor phase deposition processes, including metal oxides having use in semiconductor device applications, residual stresses in the deposited thin film material can render the film deficient or even useless for its intended function. In other cases, certain amounts of a tensile or compressive stress are beneficial.
Such residual stresses may for example interfere with desired properties of the product film, such as piezoelectric, paraelectric, superconducting, ferroelectric, and memory (in the case of memory alloy thin films) properties, and the residual stresses may compromise the structural integrity of the material, particularly in applications where the thin film is subjected to differential thermal effects or mechanical impact, vibration, etc.
For these reasons, it is desirable to form thin film materials in such manner as to minimize and/or otherwise control the residual stress in the product material.
The prior art has attempted to minimize the residual stresses in thin film materials by focusing largely on the selection and manipulation of the process conditions, using film-forming methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), spin-on formation, etc.
In addition to such process conditions approach, there has been recognition that the mechanical fixturing and physical set-up of the substrate element on which the thin film is formed may have significant impact on the stress state of the final product film.
For example, in discussing the highly compliant nature of PLZT (Pb--La--Zr--Ti--O), Haertling (G.H. Haertling, Ferroelectrics, 1987, Vol. 75, pp. 25-55, III. Properties, 2. Mechanical Properties, 2nd paragraph) has mentioned a device reported in J. Maldonado and A. Meitzler, Proc. IEEE, 1971, Vol. 59, p. 368, in which "domain orientation was accomplished by mechanically flexing a PLZT plate bonded to a plexiglas substrate." This work evidences the fact that stress can be used to control domain orientation.
The present invention is therefore directed to an improved methodology to control the stress in thin film materials formed on a substrate, and to correspondingly improved thin film materials having enhanced stress characteristics relative to the films produced by the prior art.